1. Technical Field
This invention relates to digital televison (DTV) and, more particularly, to use of an overlay signal in the DTV transmission for brevity signaling for public safety.
2. Background Art
High definition television (HDTV) is an emerging technology that is capable of providing service either in analog or digital format. In the United States, research on HDTV has focused on digital, rather than analog, technology. While digital HDTV is not currently available, Japanese companies have developed an HDTV system based on analog technology (known as Hivision) that has been in use since 1991. Because of the potential advantages of digital HDTV and many technical problems shared by both types of systems, research in digital HDTV has also been active in Japan. See, for example, David K. Kahaner in xe2x80x9cHDTV Research in Japanxe2x80x9d, IEEE Micro, Oct. 1993, pp. 49-53.
One of the most important prevalent problems in DTV is that of multipath. In fact, it is useful to think of the DTV channel as multipath limited and not power limited. Multipath may arise from fixed structures acting as reflectors in the transmission channel, such as building walls. Moving objects, such as airplanes, may also cause a multipath condition. Even microreflections in cabling can cause multipath. See, for example, P. T. Marhiopoulos and M. Sablatash, xe2x80x9cDesign of a Ghost Canceling Reference Signal for Television Systems in North Americaxe2x80x9d, Proceedings of Canadian Conference on Electrical and Computer Engineering, Vancouver, BC, Canada, Sep. 14-17 1993, pp. 660-663.
It has been proposed, as described in commonly assigned copending patent application Ser. No. 09/201,376, filed Nov. 30, 1998, that the DTV transmitter overlay a wideband signal in order to provide a way to characterize the multipath limited channel in order to mitigate multipath effects. This signal would be processed by receivers equipped to do so, but would not adversely affect the picture produced by receivers not so equipped; that is, the technique is backward compatible.
The Emergency Alert System (EAS) is a valuable government service that provides timely and crucial warning to customers of public broadcast media. As recently as 1997, the new EAS system replaced the older Emergency Broadcast System (EBS). The upgraded system allows for less obtrusive testing and converts many of the its messages into digital formats. As digital television (DTV) matures, it will provide a challenge to make better use of the publicly owned spectrum to better serve the public. One important way to do this is to upgrade communications for public safety and the commonweal to even higher levels.
A brevity signaling module for a digital television (DTV) receiver enables reception of emergency broadcast messages. The overlay signal received at the receiver is squared, producing spectral lines at DC and twice the overlay signal frequency, or 2xcfx89, where xcfx89=2xcfx80f and f is the center frequency of the overlay signal. The squared signal is analyzed to determine which of different frequency tones, 2xcfx89, 2xcfx891, 2xcfx892, . . . , 2xcfx89n, is present, where the different frequency tones 2xcfx891, 2xcfx892, . . . , 2xcfx89n are assigned to different functions. The detected tone is used by a control module to determine whether normal multipath processing should be performed or whether this processing should be suppressed and auxiliary functions performed, including turning on the DTV receiver power in order that an emergency broadcast message might be conveyed to persons in the vicinity of the DTV receiver.